Voltage-stepdown circuit arrangement for television power supply

ABSTRACT

In order to provide a reduced output voltage for feeding a transistorized load, a rectified supply voltage from an AC utility outlet is chopped under the control of a recurrent pulse in the load circuit, specifically the flyback pulse of the horizontal sweep circuit of a television receiver, with the aid of a normally blocked transistor connected to be unblocked by an intermittent biasing current from a secondary winding of a transformer whose primary winding acts as an inductance of a smoothing network for the chopped supply voltage.

358-190. OR 395949499 SR unlten males ratent [in 3,594,499

[ 72] Inventors Vineenzo Sansone; [56] References Cited I [2] l A I Ngag gg M Pam, UNITED STATES PATENTS pp o. Filed Feb. 27 1969 3,5 l0,5785/1970 Bazln l78/7.l [45] Patented July 20, 197 1 PrimaryExaminer-Richard Murray [73] Assignee Ates Componenti Eleetronici S.P.A.AltorneyKarl F. Ross Milan, Italy [32] Priority Mar. 8, 1968 331 ItalyABSTRACT: In order to provide a reduced output voltage for E Sm ufeeding a transistorized load, a rectified supply voltage from [54]VOLTAG wN CIRCUIT ARRANGEMENT an AC utility outlet is chopped under thecontrol of a recur- SUPPLY rent pulse in the load circuit, specificallythe flyback pulse of the horizontal sweep circuit of a televisionreceiver, with the [52] US. Cl l78/7.3 R, aid of a normally blockedtransistor connected to be un- 178/DlG. 1 1 blocked by an intermittentbiasing current from a secondary [5 1] Int. Cl. H04n 5/00 winding of atransformer whose primary winding acts as an in- 178/6 PS, ductance of asmoothing network for the chopped supply volt- 7.1, 7.2, 7.3, 7.5 age.

[50] Field of 3 CH v s v 1 M T 8 A A C in C1 c2 r- 4' C 3 i 22 15 7 m lE l 11 Chopper [3: VS" s' 71 s" v To CRT 1 mm W Anode OW "7 W A c7 Ly 5Sweep Conrro 7Z2 Horizontal 5c Deflccf ion Yoke memsmzonn 3,594,499 sum2 M2 \Vc y ('00 4 (i) o I q I v v Q I v f I Q I (C) 0 s I o [b (5)0 n nJ Q v r N/ V (=00 N J n mvEN'mRs: t I 1!, t Vince'nzo Sansone FrancoGaffi FIG. 2 BY {Karl R ss Attorney VOLTAGE-STEPDOWN CIRCUIT ARRANGEMENTFOR.

TELEVISION POWER SUPPLY Our present invention relates to a power supplydesigned to step down the voltage of an available. AC or DC source ofelectric energy, such,as-an'outlet= from the utility mains of abuilding, for feeding a low-voltage direct-current load.

In a television receiver, for example, voltages of notmore. 0

'voltage-step-down circuit arrangement for; the purpose described whichreduces volume, weight and cost of a power supply incorporating same andwhich is particularly adapted for transistorized electronic equipmentsuch as television receivers. v

A more specific object is to provide a power supply network utilizing,for the most part,' components conventionally present in a televisionreceiver for the purpose of generating a low-voltage direct-currentsupply..

These objects are realized, pursuant to our present invention, by theprovision of a chopper in the form of one or more normally blockedtransistors inserted between an input circuit and a load circuit, incombinationwithcontrol means for periodically unblocking the transistoror transistors to generate a pulse train which is then integrated by animpedance pathin the transistor output. With the input circuit connectedto a source of substantially constant unipolar or alternating voltage(and including the usual rectifying and smoothing circuitry in thelatter case), the integrated output voltage has a magnitude whichisafraction of the level of the synchronizing pulses applied to ascanner of a television tube,

preferably to the horizontal sweep circuit thereof; this scanner iscoupled to the integrating circuit in the chopper output so as tounblock the chopping transistor or transistors during a specific phaseof each scanning sweep, advantageously during the flyback stroke. Thus,the horizontal sweep circuit associated with a deflecting yoke of acathode-ray tube may in- 1 nection oftheir-main electrodes (usually,emitters and collectors-)2 The invention will be described in greaterdetail with reference to the accompanying drawingin which:

FIG. 1 is a circuit diagram of'a power supply network representinganillustrative embodiment; and

' FIG. Zisa set of graphsrelative to the operation of the networkof HO:1.

The supply networkshown in' FIG. lcompriscs a source of i"altematingiinput-voltage V constituted; for example, by autilityoutletof220 v; r.m'.s. This voltage is rectified in a cir-'cuitsh'own-to include an input resistorRg, (which may-form part'of theinternal resistance of the source), a diode DI and an RC networkcomposed of a series resistor R1 and two shunt condensers CI; C2; Theoutput terminal B of 'thi's'network is connected to groundthroughaseries circuit including a condenser C3,a resistor R2-and a furthercondenser C6jth e' latter being rshunted by'a load resistor R,; acrosswhich a reduced output voltage-V is developed. Point B is also connectedto i the emitter of a first transistor Q1, shownto beof the PNP" type,whose collector is tied to the emitter of a similar second transistor02; the output terminal C of transistor Q2, joined to the-collectorthereof, is connected through a ballast resistor" -R8-to a point"A'whence the primary "winding For a trans-- former Trl extends to thejunction -D of condenser C6 and resistor R2.

Transformer 'Trl has two secondaries S, S"respectively connected acrossthe emitter and base electrodes of i transistors Q1 and O2 in'serieswith'respectivediodes D3, D3"and condensers C4, C4"; the lattercondensers form part i of-two identicaldifferentiationcircuits'including respective input voltage as determinedby the. frequency and direction of pairs of shunt resistors R3, R4 andR3", R4. A further secondary winding S of transformer Trl supplies'astepped-up voltage, by way of a rectifying circuithere simplyrepresented i by a diode-D4, to a high-voltage anode of an associatecathode-ray tube not'furthe'r illustrated, this tube being providedwith'the usualscanning meansincluding a horizontal 5 sweep-controlcircuit SCand' an electromagnetic yoke simply 'shown as acoil L,,. I

Sweep-control circuit SC'comprises a storagecondenser C7 connected, inseries with coilL between ground and junction A, this series combinationbeing further shunted bya flyback condenser C8 of substantially lowercapacitance than condenser C7, a diodeDZ, and the emitter-collectorcircuit of an auxiliary transistor Q3 whose base and collectorperiodically receive synchronizing pulses sp .froma source not shown bythe retrace period,,this voltage pulse being transmitted to the chopperthrough the inductively coupled primary and secondary windings referredto. ln orderto limit the duration of the output pulse of the choppertosubstantially the length of a retrace period, we prefer to insert adifferentiation circuit between the transistor input and-the associatedtransformer secondary.

I Conveniently, the transformercontrolling the chopper may be acomponent conventionally provided in a television power supply fordelivering a stepped-upsvoltage to a high-voltage electrode of thereceiver, such as the final accelerating anode of the picture tube. Inthis case, only a few turns of winding need to be added to such apreexisting transformer to drive the chopper.

Although, in principle, the chopper need only include a singletransistor, the use of several (e.g., two) such devices in cascadeenables the utilization of relatively cheap commercial transistors sincethe voltage drop developed thereacross during cutoff will becorrespondingly reduced by the series conway of a transformer Tr2.

Reference will now bemade'to FIG. 2 for a description of I the=modeofoperationof the chopper CH and the sweep-control circuit SC of HO! 1.

When the chopper is first connected across source V5,, e. g., via amanual switch not shown,the resulting voltage surge at a will presentlybecome apparent, the condensers C7 and C8 are periodically rechargedthrough thechopper Cl-l during steadystate operation.

At a time t FIG. 2, the electronic switch constituted by transistorQ3lis assumed to'be' closed by an unblocking pulse sp, graph (a-). Acurrent i graph '(b), passes at this instant through yoke L; inthe'direction indicated by an arrow in FIG. 1 (taken as positive),thereby discharging the condenser C7 through transistor 03 at a ratedetermined by the resonance frequency of the tuned series circuit L,,,C7. At instant 1,, pulse sp terminatesso that transistor 03 cuts offwhereupon the yokecurrent i, flows into condenser C8 which charges up ata rate depending on the resonance frequency of series reactances L, C8(it'being assumed that the capacitance of condenser C7 is so much largerthan that of condenser C8 as to have only a negligible effect upon thecharging of the latter). This charging of condenser C8, by a currentflow i,also

shown in graph (c) of FIG. 2, begins to raise the potential of point Awhereby a current surge i through primary P is initiated, the directionof this surge (arrow in FIG. 1) being here taken as positive; see alsograph (d) of FIG. 2. This surge, in turn, gives rise to a secondarycurrent i,', i," in each of windings S, S", as indicated at i, in graph(e) of FIG. 2. Differentiation of this current flow by theaforedescribed circuits gives rise to base currents i i, alsoillustrated at i,, in graph (1) of FIG. 2. The resultantflow ofcollector current i through transistors Q1 and O2 in series has beenplotted in graph (g). This current flow, in turn, sharply raises thepotential V of point C and entrains a similar rise in the potential V Aof point A as indicated ingraphs (h) and (i), respectively.

At instant t the flyback current i, stops, primary current i, reverses,secondary current i ceases and the base current 1', goes negative. Owingto the finite sweepout time of transistors 01 and Q2, however, the flowof collector current i continues for a short period, terminatingapproximately at time t,. During the interval t i condenser C8 hasdischarged through inductance I. and capacitor C7, the flow of negativeyoke current 1', continuing beyond instant t by way of diode D2 whichbecomes conductive as soon as point A is driven negative,

7 with reference to ground, by the inverted flyback current. At

this stage, however, the yoke current is again controlled by thereactances L, C7 to the exclusion of the capacitance C8 which has beenshort circuited by the diode D2. Thus, as illus' trated in graph (b)ofFIG. 2, the substantially linear rise in yoke current is more gradualthan its descent during the flyback interval 1,4,.

At some point between time I and the instant I, when current 1, goes tozero, a new synchronizing pulse sp appears in the input of switchingtransistor 03. The resulting unblocking of this transistor takes effect,however, only upon the reversal .of the yoke current at the instant t,whereupon this current again flows through the transistor as previouslydescribed for the instant t the cycle being then repeated. The flow ofyoke current i,, through the transistor Q3 has been indicated in graph(b) by vertical shading.

lmpedances P, C6 and R represent an integrating circuit whichsubstantially maintains the load voltage V i.e., the potential of pointD, at an average level of, say, 30 v. as also indicated in graph (d) ofFIG. 2. With the potential of point B he ld n approximately 280 v., theresistor R8 may be dimen- -"s'ioned to establish the peak of voltagepulse V at about 250 v.

It will be noted that the potential of point A, graph (1), is notchanged during the forward sweep of the beam in the period t so that thelinearity of that sweep inherent in the design of the control circuit SCis not affected.

The output voltage V may be used to control the vertical beamdeflection, to energize the generator of synchronizing pulses sp and todrive other equipment in the audio or video channels of the televisionreceiver containing the aforementioned cathode-ray tube. Naturally,transformer Trl may have additional secondary windings leading to otherloads and, if desired, inductances similar to coil L may be energized inparallel therewith or may be included in other series-resonant circuitsbranched across condenser C8.

We claim:

1. A power supply network for converting a continuous supply voltageinto a reduced continuous output voltage, comprising an input circuitconnected to receive said supply voltage, a load circuit, circuit meansincluding normally blocked transistor means connecting said inputcircuit across said load circuit, and control means for periodicallyunblocking said transistor means to generate a pulse train in the outputthereof, said circuit means further including impedance means in cascadewith said transistor means for integrating said pulse train, saidimpedance means comprising a primary winding and said control meanscomprising a secondary winding inductively coupled to said primarywinding.

2. A network as defined in claim 1 wherein said windings are part of atransformer also having a voltage-stepup load winding, said load circuitincluding a high-voltage electrode connected to be ener ized from saidload winding.

g 3. A network as de med in claim 2 wherein said load circuit comprisesa part of a television receiver with a cathode-ray tube provided withsaid electrode.

4. A network as defined in claim 1 wherein said load circuit comprisespart of a television receiver provided with a sweep circuit including asource of synchronizing pulses, said primary winding being coupled tosaid sweep circuit for energization in the rhythm of said synchronizingpulses.

'5. A network as defined in claim 4 wherein said sweep circuit comprisescoil means, a storage condenser in series with said coil means, aflyback condenser of lower capacitance than said storage condensershunting the series combination of said storage condenser and said coilmeans, unidircctionally conductive electronic switch means bridgedacross said flyback condenser and connected to said source forperiodically establishing a discharge path for said storage condenser byway of said coil means, and diode means connected in parallel with saidswitch means but with opposite polarity to give passage to a return flowrecharging said storage condenser at a rate determined by the inductanceof said coil means, said sweep circuit being connected across the outputof said transistor means in parallel with said impedance means forenergizing said primary winding in response to a voltage pulse developedacross said flyback condenser.

6. A network as defined in claim 5, further comprising a resistor in theoutput of said transistor means connected in series with both saidimpedance means and said sweep circuit.

7. A network as defined in claim 5, further comprising a reactivestarting circuit bypassing said transistor means and extending from saidinput circuit to said condensers for initially charging same uponapplication of said supply voltage thereto.

8. A network as defined in claim 5 wherein said control means furthercomprises a differentiation circuit inserted between said secondarywinding and said transistor means for biasing the latter intoconductivity during only an initial part of said voltage pulse.

9. A network as defined in claim 1 wherein said transistor meanscomprises a pair of substantially identical transistors connected incascade.

1. A power supply network for converting a continuous supply voltageinto a reduced continuous output voltage, comprising an input circuitconnected to receive said supply voltage, a load circuit, circuit meansincluding normally blocked transistor means connecting said inputcircuit across said load circuit, and control means for periodicallyunblocking said transistor means to generate a pulse train in the outputthereof, said circuit means further including impedance means in cascadewith said transistor means for integrating said pulse train, saidimpedance means comprising a primary winding and said control meanscomprising a secondary winding inductively coupled to said primarywinding.
 2. A network as defined in claim 1 wherein said windings arepart of a transformer also having a voltage-stepup load winding, saidload circuit including a high-voltage electrode connected to beenergized from said load winding.
 3. A network as defined in claim 2wherein said load circuit comprises a part of a television receiver witha cathode-ray tube provided with said electrode.
 4. A network as definedin claim 1 wherein said load circuit comprises part of a televisionreceiver provided with a sweep circuit including a source ofsynchronizing pulses, said primary winding being coupled to said sweepcircuit for energization in the rhythm of said synchronizing pulses. 5.A network as defined in claim 4 wherein said sweep circuit comprisescoil means, a storage condenser in series with said coil means, aflyback condenser of lower capacitance than said storage condensershunting the series combination of said storage condenser and said coilmeans, unidirectionally conductive electronic switch means bridgedacross said flyback condenser and connected to said source forperiodically establishing a discharge path for said storage condenser byway of said coil means, and diode means connected in parallel with saidswitch means but with opposite polarity to give passage to a return flowrecharging said storage condenser at a rate determined by the inductanceof said coil means, said sweep circuit being connected across the outputof said transistor means in parallel with said impedance means forenergizing said primary winding in response to a voltage pulse developedacross said flyback condenser.
 6. A network as defined in claim 5,further comprising a resistor in the output of said transistor meansconnected in series with both said impedance means and said sweepcircuit.
 7. A network as defined in claim 5, further comprising areactive staRting circuit bypassing said transistor means and extendingfrom said input circuit to said condensers for initially charging sameupon application of said supply voltage thereto.
 8. A network as definedin claim 5 wherein said control means further comprises adifferentiation circuit inserted between said secondary winding and saidtransistor means for biasing the latter into conductivity during only aninitial part of said voltage pulse.
 9. A network as defined in claim 1wherein said transistor means comprises a pair of substantiallyidentical transistors connected in cascade.